The present invention relates, in general, to a method and to apparatus for determining the distance and direction from a bore hole, or well being drilled, to a target well, and more particularly to apparatus for generating a source magnetic field within a bore hole and for measuring distortions in that field caused by a nearby target well casing.
It is well known that in drilling deep wells for oil and gas exploration, precise control of the path followed by the drill is extremely difficult, so that it is virtually impossible to know the exact location of the well at a given depth. For example, a drilling tolerance of .+-.1/4 of a degree will allow the bottom of a 10,000 ft. well to be positioned anywhere within a circle 87 ft. in diameter, and numerous factors can increase this deviation. This is not of particular concern in normal drilling operations, but if an accident should occur, such as a rupture in the casing of the well which causes the well to blow out, it may become necessary to plug the well at or below the point of rupture to bring it under control. In order to do this, a relief well must be drilled to intersect the original well at the desired level, and since such blow-outs produce extremely hazardous conditions at the surface in the vicinity of the original well, the relief well must be started a considerable distance away and drilled at an incline in order to protect the personnel and equipment used.
Because the same problems of control of the direction of drilling are encountered in the relief well as were encountered in the original well, the location of the relief well bore cannot be determined with precision; accordingly, it is extremely difficult to determine the distance and direction from the relief well to the blown-out well. A further difficulty is that in some cases the force of the explosion resulting from the rupturing of a well casing can do so much damage at the surface that it may be difficult to determine exactly where the wellhead is, much less where the well is located at a depth of 10,000 ft. or more. Because of the danger at the wellhead, the relief well is started 1/2 mile or more away from the area of the original wellhead, angled toward the original well until it approaches relatively closely, and then follows the original well to the location where the wells are to intersect. Thus, the relief well usually is very complex, compounding the problem of knowing exactly where it is located with respect to the original well.
In addition to the problems encountered in drilling wells so that they intersect at specified levels to relieve blow-out problems, the inverse problem is also very important. That is, it is often extremely important to be able to drill wells so that they do not intersect each other. This is a particularly difficult problem which arises most often when multiple wells are drilled from a single drilling platform in offshore as well as in urban environments.
For example, in some offshore platforms, as many as 72 wells are drilled on 6 ft. centers from a single platform and it is extremely important that there be no accidental well intersections when drilling in such crowded circumstances.
Numerous systems and techniques have been developed in the prior art for determining not only the presence of a target well, but for determining with some degree of accuracy its general direction. Examples of such prior devices are found in U.S. Pat. application Ser. No. 203,912 of Arthur F. Kuckes, filed Nov. 4, 1980, now U.S. Pat. No. 4,372,398, issued Feb. 8, 1983, entitled "Method of determining the Location of a Deepwell Casing by Magnetic Field Sensing", and in the prior art discussed in that patent. However, it has now been found that potentially serious problems are encountered when routine use of such prior devices is contemplated for ascertaining the exact distance to adjacent wells, for the parameters relied upon in those prior systems are now found to vary considerably and to adversely effect the accuracy of measurements made by such systems.
Some prior art systems and methods rely on the remanent magnetism of the casing in a target well and use highly sensitive magnetometers to detect that magnetism and to determine the distance to the well. Gradiometers are used in conjunction with such devices to eliminate the effect of the earth's magnetic field, which is assumed to be a constant. Other devices rely on the conductivity of the earth and of the casing to permit the flow of electric currents which are used in detecting the location of the target well.
It has been found, however, that the remanent magnetism in target casings is highly variable since the the casings take on magnetization by induction in the earth's magnetic field, and because of the diverse nature of the magnetizing factors and the histories of the materials which are used in the casings. Such variations in the remanent magnetism make it extremely difficult to ascertain with any accuracy the exact distance from a magnetic field sensor to the target well. It has also been found that the variations in the earth's conductivity, the effect of a concrete sheath on the target well, and other similar factors also make accurate measurements difficult, and accordingly the results obtained by such prior systems cannot be relied upon with any degree of safety.
A further problem lies in the fact that many of the sensing devices proposed in the prior art require removal of the drill string from the well being drilled before the logging equipment can be lowered into position for measurement. This is a time consuming and expensive task, and in the case of a runaway well, allows the serious consequences of such an accident to continue unabated for an excessive amount of time.